J.R. Dawson
When steam is used for the partial sterilisation of soil, the final temperature of the soil is 212°F which is the saturation temperature of steam at atmospheric pressure. When the soil is heated to 212°F, practically the whole of the soil population is killed except the ammonifying bacteria, and when the soil cools these release large quantities of ammonia from the organic matter in the soil making it an unsuitable medium for plant growth. In some glasshouse soils another problem, which is inhibiting to plant growth, is the release of large quantities of water soluble and exchangeable manganese when the soils are heated.

In 1952 Morris (1) suggested that if the final temperature of the heated soil was limited to 180°F it might be permissible to put plants into the soil within a few days of heat treatment taking place and that a method of achieving this would be to mix air or other gas with steam. Work by Baker (2) and Baker and Olsen (3) has shown that micro-organisms which are parasitic on plant roots are generally killed at lower temperatures than saprophytic organisms, and that an optimum heat treatment temperature might be as low as 140°F. This would be only slightly injurious to the soil chemically and would not kill beneficial organisms.

When steam is mixed with air at or near atmospheric pressure the mixture attains a dewpoint temperature between that of the steam and that of the air. Because some of the steam is condensed in heating the air the result is a slightly wet saturated mixture the temperature of which is dependent on the ratio of the air and steam as shown in fig. 1.

A simple and inexpensive method of producing steam-air mixtures is to use the venturi principle for mixing atmospheric air with steam. To save fuel Bunt (4) used similar apparatus to partially sterilise glasshouse soils at a temperature of 180°F. The principle has also been used in Australia and in U.S.A. both by Baker and several growers.

The current research programme on the heat treatment of soil with mixtures of steam and air is being carried out jointly by the N.I.A.E. and the Glasshouse Crops Research Institute. Significant biological and chemical advantages of the use of steam-air mixtures have been shown in a number of plant experiments at G.C.R.I. (5) (6).

Dawson, J.R. (1968). HEAT TREATMENT OF SOILS AT TEMPERATURES BELOW 212°F. Acta Hortic. 7, 129-137
DOI: 10.17660/ActaHortic.1968.7.14

Acta Horticulturae